A rather recent addition to laws designed to reduce these numbers was the adoption of compulsory hands-free devices for mobile phones. Their safety value is easy to understand. When you look at a mobile phone display you cannot simultaneously look at the road. Similarly, using your hands for typing and using them for steering are at least partly incompatible actions.

How mobile phone use impairs sight and hands.

From a psychological point of view the current law tries to ensure that visual input channels (eyes) and motor output channels (hands) remain undisturbed. But what about the brain areas which control these channels?

This is the question recently investigated by Bergen from UC San Diego and colleagues. They put undergraduates in a driving simulator giving the impression of a motorway with steady traffic and a car in front of the driver breaking from time to time. Simultaneously, the driver had to judge simple true/false statements from the motor domain (e.g., “To open a jar, you turn the lid counterclockwise.”), the visual domain (e.g., “The letters on a stop sign are white.”), or the abstract domain (e.g., “The capital of North Dakota is Bismarck.”). As a baseline condition, people were just asked to say “true” or “false” several times.

Why choose such questions? There is both behavioural and brain-imaging evidence that language comprehension involves the simulation of what was said. This set of findings is often summarised as embodied cognition and its take-home message is something like this: in order to understand it, you mentally do it. For example, to answer a motor question, you use your brain areas doing motor control and make them simulate what it would be like to open a jar. Based on the outcome of this simulation you answer the question.

So, will visual or motor questions affect driving differently than abstract questions because the former engage the same brain areas as those needed for driving while the latter don’t? The alternative would be that asking anything distracts because general attention gets pulled away from driving.

The results go both ways. First, one measure was affected by the true/false statements but not by which kinds: quickly breaking when the car in front breaks. The time it took to do so was longer if any sort of question was asked compared to baseline. This suggests that domain general mechanisms were interfered with through language, e.g., attention.

Was she a safe driver? May depend on whether she talked and if so about what.

Second, one measure was affected by what kind of statements had to be judged:generally holding a safe distance to other cars. This distance was greater if visual questions were asked compared to abstract questions and compared to baseline. A similar, albeit not as clear, pattern emerged for motor questions. It looks as if participants were so distracted by these kinds of questions that they fell behind their optimal driving distance. This suggests that a task such as keeping a safe driving distance which requires visual working memory (compare ideal distance to actual distance) and corrective motor responses (bring ideal and actual distances closer together) is influenced by language comprehension through mental simulation.

On the one hand, the scientific implications are quite straight forward. Bergen and colleague’s results suggest that those low level perception and action control areas which are needed for quick reactions are not what embodied cognition is about. Instead it seems like embodied cognition happens in higher perceptual and motor planning areas. Furthermore, the whole embodied cognition idea gets quite a boost from a conceptual replication under relatively realistic conditions.

On the other hand, the practical implications are somewhat controversial. Because talking in general impairs quick reactions by the driver, even hands-free devices pose a risk. This danger is compounded by talking about abstract topics since the driving distance is reduced compared to visual topics.

The authors refrain from saying that any sort of conversation should be prohibited. Passengers share perceptual experiences with the driver and can adjust their conversations to the dangerousness of the situation. Mobile phone contacts can’t do this. But what if you want to be really really safe? Well, cut your own risk of dying and take public transport. There you can chat and cut your death risk by 90% (bus) or even 95% (train or flight) compared to car travel (EU numbers).